The existence of the dark field microscope is common knowledge. The microscope includes a light source for emitting illumination light, a light collecting optical system including a light collecting side condenser lens for collecting the illumination light from the light source to illuminate an observation sample, and an image forming optical system including an objective lens for receiving scattered light from the observation sample to form a magnified image of the sample.
FIG. 1 is an explanatory diagram illustrating a principle of a conventional general dark field microscope.
Light from a light source is illuminated on an observation sample (21) in a flow cell (20) as circular illumination light (10) through a light collecting side condenser lens having a high numerical aperture. The illumination light is scattered by the sample (21); however, the illumination light (11) itself is shielded by an aperture diaphragm (23) present within an objective lens (22). For this reason, only scattered light (12) is emitted from the objective lens (22). By collecting the scattered light on an imaging area of a camera, a dark field image of the sample (21) can be obtained.
Light is scattered even by a particle or a structure smaller in size than a wavelength thereof, and therefore if a dark field microscope is used, even an object having a size equal to or smaller than a wavelength of visible light can be observed.
For example, a diameter of a microtubule is as small as approximately 24 nm; however, it becomes visible by a dark field microscope having a mercury lamp as a light source.
The dark field microscope has additional advantages, for example, a device and an operation are relatively simple; a high contrast image can be obtained; a raw sample can be observed without preprocessing; supermolecular movement and morphology change can be dynamically observed; and the like.
A conventional technique relating to the dark field microscope includes ones described in Patent documents 1 to 4.
Patent document 1 relates to an illumination system capable of easily switching between a bright field and a dark field; Patent document 2 relates to a configuration capable of observing only a target micro object without reducing a resolution; Patent document 3 relates to a configuration capable of observing an extremely fine object such as a flagellum of a living bacterium; and Patent document 4 relates to high speed processable automatic focusing and increase in brightness in the dark field microscope.    Patent document 1: Japanese Unexamined Patent Publication No. H09-297266, “Microscope”    Patent document 2: Japanese Unexamined Patent Publication No. H09-15507, “Dark field microscope”    Patent document 3: Japanese Unexamined Patent Publication No. H08-122651, “Transmission dark field microscope”    Patent document 4: Japanese Unexamined Patent Publication No. H05-346532, “Automatic focusing device of microscope and dark field microscope”
The dark field microscope according to such the conventional technique requires the aperture diaphragm (23) within the objective lens (22), and is also subject to the constraint that the light collecting side condenser lens having a high numerical aperture should be used.
If illumination can be provided through a long focal distance condenser lens having a low numerical aperture, a space can be ensured above the flow cell (20), and therefore there arise advantages of expanding an application range of experimental observation, and the like. However, a dark field microscope capable of realizing this has been absent.